Now, we present the first result of this section. This is a well known and
very important property concerning the Navier boundary conditions (see Lemma
4.1 and Corollary 4.2 in [1]}).  Let \(k\)  be the curvature of \(\Gamma \) . Parameterizing \(\Gamma \)  by arc length \(s\) , the following relation holds
\(\tfrac{\partial \mathrm {n}}{\partial \mathrm {\tau }}=\tfrac{d\mathrm {n}}{ds}=k\mathrm {\tau }.\) 
